Objective: To determine the reliability of ST-segment interpretation by paramedics from lead-II rhythm strips obtained in the prehospital setting.
Design: Prospective, blinded study of 127 patients transported by an urban/rural emergency medical services system with complaints consistent with ischemic heart disease.
Methods: Emergency department physicians asked emergency medical technician-paramedics (EMT-P) via radio to evaluate ST-segments for elevation or depression and grade it as "mild," "moderate," or "severe." Then, this rhythm strip was interpreted blindly by emergency physicians who also interpreted the lead-II obtained from a 12-lead electrocardiogram (ECG) obtained in the emergency department (ED). The field interpretation was compared with the subsequent readings and the final in-patient diagnosis using positive predictive value (PPV), negative predictive value (NPV), and the Kappa statistic. Markedly discrepant interpretations were analyzed separately.
Results: Using physician interpretation as the reference standard, paramedic interpretation of the lead-II ST-segments obtained in the prehospital setting was correct (within +/- 1 gradation) in 113 out of 127 total cases (89%). Of 105 patients for whom final hospital diagnosis was available, the ST-segment on the rhythm strip obtained in the prehospital setting, had a positive predictive value of 74% and a negative predictive value of 85% for myocardial ischemia or myocardial infarction (MI) (p < 0.001, Kappa = 0.59). Discordant interpretations between the paramedics and emergency physicians often were related to a basic misunderstanding of rhythm strip morphology.
Conclusion: Field interpretation of ST-segments by paramedics is fairly accurate as judged both by emergency physicians and correlation with final patient outcome, but its clinical utility is unproved. A small but clinically significant number of outliers, consisting of markedly discrepant false positives, reflects paramedic uncertainty in identifying the deviations of the ST-segment.